The present invention relates to a novel organic electroluminescent material and the improved organic electroluminescent devices prepared therefrom. More specifically, the present invention relates to a novel family of organic compounds for use in organic electroluminescent devices which not only provide enhanced electroluminescence in the blue color range so that they can be advantageously used as the light emitting layer of the organic electroluminescent device, but which also provide a hole injection capability so that they can also be used as the hole injection layer. By merging the blue light emitting layer with the hole injection layer, the present invention allows the blue light emitting organic electroluminescent devices to be simplified, while, at the same time, it also improves the blue emitting efficiency of the organic electroluminescent devices so constructed.
Organic electroluminescent devices are becoming an increasingly important category of thin film organic semiconductor devices. An organic electroluminescent device operates by applying an external electric field, wherein electrons enter from the cathode and holes enter from the anode, of the organic electroluminescent device. When the electrons and the holes meet after they travel through the electron transport layer and the hole transport layer, respectively, energy will be generated in the form of light (luminescence). This is a short-hand description of the organic electroluminescent phenomenon. Organic electroluminescent devices provide the advantages of self-luminescence, wide viewing angle, high response rate, simple fabrication, and low power consumption, and are thus excellent candidates for use in flat panel displays.
Typically, the organic electroluminescent devices have a multi-layered structure, comprising an hole injection layer, a light emitting layer, and an electron injection layer. The hole injection layer and electron injection layer also provide the functions to transport holes and electrons, respectively. This is illustrated in U. S. Pat. No. 5,151,629.
U.S. Pat. No. 4,539,507 discloses an electroluminescent device which comprises, in sequence, an anode electrode, a hole-injecting zone, an organic luminescent zone, and a cathode electrode, at least one of the electrodes being capable of transmitting at least 80% of radiation having wavelengths longer than 400 nm. The organic luminescent zone comprises an electron-transporting compound that provides a maximum electroluminescent quantum efficiency of at least about 5xc3x9710xe2x88x924 photons/electron. The hole-injecting zone consists essentially of 1,1-bis-(4dipolylaminophenyl)-cyclohexane, and the hole-injecting zone and the luminescent zone having a combined thickness of no more than 1 micron. The cathode is an indium cathode. Typically, as taught in the ""629 patent, a separate luminescent layer is provided separately from the electron-transporting layer, in order to allow maximum choice for each individual layer and achieve best luminescence efficiency.
Another important requirement in the development of organic electroluminescent devices is to develop RGB (red, green, and blue) light emitting devices so as to satisfy the need of a color flat panel display. At the present time, green organic electroluminescent devices have seen the most successful development. Examples of prior art references that teach green organic electroluminescent devices include U.S. Pat. Nos. 4,769,292 and 5,227,252. In both references, green organic electroluminescent devices with a half-time life of more than 10,000 hours and a luminescence of 1,000 cd/m2. Red organic electroluminescent devices are discussed in U.S. Pat. Nos. 5,409,783 and 5,432,014. However, at the present time, the wavelength of luminescence emitted by these devices do not satisfy the CIE standard specified for red light.
U.S. Pat. Nos. 5,077,142 and 5,389,444 taught blue organic electroluminescent devices. However, the luminescence of blue light from these devices are only about 100 cd/m2. At such a low luminescence, these devices have very limited practical use.
Because of the increasing popularity and thus importance of flat panel displays, it is highly desirable to develop new organic compounds that can be advantageously used in the low power consumption organic electroluminescent devices which can emit luminescence especially in the red and blue color spectra. At the present time, there is a lack of good organic electroluminescence compounds that will satisfy this need.
The primary object of the present invention is to develop an improved organic electroluminescent device in the blue color range. More specifically, the primary object of the present invention is to develop an improved organic compound which can provide improved luminescence in the blue color range, as well as simplify the construction of organic electroluminescent devices.
In the present invention, it was discovered by the co-inventors that, by incorporating a tertiary amine group into a organic blue luminescence compound, the tertiary amine imparts a hole-transportability and hole-injectivity into the organic blue luminescence compound. One of the main advantages of the discovery made in the present invention is that the addition of the tertiary amine group to the organic blue luminescence compound does not adversely affect its blue luminescence; it actually enhances it. Thus, the tertiary amine imparted organic blue luminescence compound can be used as a combined light emitting/hole transporting layer to greatly simply the construction of an organic electroluminescent device. The elimination of the hole transport layer can also reduce power consumption and improve the luminescence of the organic electroluminescent device.
In a preferred embodiment of the present invention, the tertiary amine group is imparted into a quaterphenyl compound to form the following compound: 
where R1 and R2, which can be different or the same, are hydrogen, C1-C5 alkyl, or C6-C12 aryl, and R3 is hydrogen, C1-C5 alkyl, a vinyl group, or an aryl vinyl group.
The tertiary amine group imparted quater phenyl compound shown above can be synthesized according to either of the following methods.
Method 1:
In this method, the quaterphenyl compound is first iodizied and the iodized quaterphenyl compound is then reacted with a secondary amine in a catalyzed reaction as follows: 
Method 2:
In the second method, a tertiary amine-substituted tertiaryphenyl compound is reacted with an aryl Grignard reagent in the presence of a catalyst to form a tertiary-substituted quaterphenyl compound. The second method is summarized in the following reaction: 
where X is Cl, Br, or I.
The organic blue luminescence compound of the present invention can be used in a two-layer or a three-layer configuration. Both configurations include an electron-transport layer. In the two-layer configuration, the organic blue luminescence compound doubles both as a blue luminescence layer and as a hole transport layer. In the three-layer configuration, an additional hole-transport layer is included.
Preferably, the electron-transport material in the electron-transport layer is a metal chelate, such as aluminum tris(8-hydroxyquinoline), bis(10-hydroxybenzo[h]quinolinato)beryllium; 1,3,4-oxadiazole or 1,2,4-triazole derivative or its derivative; a thiopyran sulfone or its derivative; or bis(benzimidazolyl)berylenedicarboximide; etc.
If a separate hole-transport layer is to be used in addition to the dual-function tertiary amine imparted quaterphenyl compound disclosed in the present invention, preferably, the hole-transport material in the hole-transport layer is a triarylamine, such as N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis(3-methylphenyl)-4,4xe2x80x2-diamine, N,N,Nxe2x80x2Nxe2x80x2-tetrakis-(4-methylphenyl)-(1,1xe2x80x2-biphenyl)-4,4xe2x80x2-diamine, N,Nxe2x80x2-bis-(1-naphthyl)-N,Nxe2x80x2-diphenyl-1,1xe2x80x2-biphenyl-4,4xe2x80x3-diamine, or 4,4xe2x80x2,4xe2x80x3-tris(3-methylphenylpnehylamino)triphenylamine, etc. On the other hand, the tertiary amine imparted quaterphenyl compound can also be used as a hole-transport material in a multi-layered organic electroluminescent device which uses other light emitting materials.
In preparing the organic electroluminescent devices, all the organic layers can be deposited by sputtering under a vacuum. Preferably, the electron-transport layer has a thickness of 50 xc3x85 to 1,500 xc3x85, more preferably 300 xc3x85 to 700 xc3x85. The hole-transport layerpreferablyhas a thickness of200 xc3x85 to 1,000 xc3x85, more preferably 300 xc3x85 to 600 xc3x85. The light emitting layer preferably has a thickness of 100 xc3x85 to 1,500 xc3x85, more preferably 200 xc3x85 to 700 xc3x85. If the tertiary amine imparted quaterphenyl compound is used as both the hole-transport layer and the light emitting layer, its preferred thickness can fall into either range. The anode material can be a high work function metal oxide, such as indium tin oxide, tin oxide, indium oxide, or zinc oxide. And the cathode material can be a low function metal, such as lithium, magnesium, calcium, beryllium, potassium, strontium, Mg:Ag alloy, or Li:Al allry. The cathode material can also be a metal compound, such as lithium fluoride, calcium oxide, strontium oxide, etc.